Rolling magnesium alloy



Patented Nov. 10, 1936 UNITED STATES ROLLING MAGNESIUM ALLOY Robert D. Lowry and Fred L. Reynolds, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich" a corporation of Michigan No Drawing. Application May 21, 1934.

Serial No. 726,755

2 Claims. (Cl. 148-'11.5)

The invention relates to improvements in methods of rolling'magneslum base alloys which contain relatively small amounts of alloying metals and in general not less than about 90 per 5 cent of magnesium. The invention is directed particularly to a method for obtaining sheet,

, plate and like products that possess a high degree of yield strength in tension. The term yield strengt herein means the load in pounds per square inch at which the stress strain curve deviates 0.2 per cent elongation from the modulus line.

The conventional practice for producing high yield strength rolled magnesium alloy sheet and the like consists in heating the metal to a temperature between about 600 and 850 F. and then repeatedly passing the same between the rolls of a mill so as to produce a reduction in thickness at each pass until the metal temperature falls to about 400 F. This procedure is repeated as often as necessary to attain the desired cumulative reduction to final thickness, the repetitions of the procedure being so regulated as to reach the ultimate thickness when the metal temperature has fallen to about 400 F.

Although magnesium alloy sheet so produce possesses relatively high yield strength in tension, the sheets are not flat, but are waved and curled, and cannot be made flat by the usual leveling operation, such as by passing the sheet at room temperature between conventional leveling rolls. If the sheet be heated and then passed between leveling rolls it can be made substantially flat, but the yield strength is materially reduced as thereby. So far as we are aware, no method is commercially available by which magnesium and magnesium alloy sheet, plate and like rolled products may be produced which possess a relatively high yield strength in tension and are substantially fiat or free from undesirable waviness.

One of the objects of our invention is to provide a method for producing from magnesium and magnesium alloys rolled products such as sheet, plate and the like possessing relatively high yield strength that are also substantially flat or free from waviness which characterizes the commercial high yield strength rolled sheet now available. Other objects and advantages will appear as the description proceeds. To the accomplishment of the foregoing and related ends, the

invention, then, consists of, the method hereinafter fully described and particularly pointed out in the claims.

Our invention is based upon the discovery that sheet, plate or like product, rolled at a temperature between about 850 and 400 F., can be made flat or level, that is substantially free from waviness, and with a relatively high yield strength in tension, if such rolled product be subjected to a heat treatment between about 600 and 850 F. l5 followed by a relatively minor amount of cold rolling, which may be carried out at a temperature between about 40 and 200 F.

In accordance with our invention, a suitable cast or extruded billet is heated to a temperature 10 between about 600 'and 850 F. and while so heated passed between the rolls of a mill so as to .produce a reduction in thickness at each pass.

Usually a reduction up to about 8 to 15 per cent or more may be produced per pass without fracl0 ture. While being thus rolled, the metal temperature falls and the amount of reduction in thickness thatmay be produced at each, pass without fracture decreases. When the metal temperature reaches about 500 or 400 F., it is 20 necessary to reheat the metal prior to further rolling and then to repeat such heating and rolling until the desired approximate total reduction in thickness is produced. The so rolled metal is then given a heat treatment which consists in 25 maintaining the same at a temperature between about 850 and 500 F. for from A to 2 hours or more, respectively, according to the temperature of the metal, since the lower the heat treating temperature the longer it must be. heated. In 30 most instances a suitable heat treating temperature is between 600 and 800 F. Following such heat treatment the metal is further rolled in a number of passes at a, temperature-between about 40 and 200 F., producing up to about 1 per cent as reduction in thickness per pass. Usually a total reduction between about 3 and 10 per cent of the metal thickness is suflicient to level the sheet and develop the desired degree of yield strength.

The following example is illustrative of.one 4 mode of carrying out the invention:-

An extruded rectangular billet of magnesium alloy containing 4 per cent of aluminum, 0.3 per cent of manganese, the balance being magnesium, was heated and rolled into sheet of a thick- 45 ness of 0.021 inch at a temperature between 780 and 400 F. in a number of passes each producing between about 5 and 15 per cent reduction in thickness per pass. The rolled sheet was then heat treated by heating the same for 1 hour at 700 F. after which it was rolled at room temperature (-80 F.) in 5 passes to a thickness of 0.020 inch. The resulting sheet was substantially level and free from waviness when laid upon a plane table. Test specimens cut from the sheets l6 produced by our method showed the following properties-tensile strength, 3438,000 lbs/sq. in.; yield strength in tension, 26-28,000 lbs/sq. in.; per cent elongation in 2 inches, 5-12 per cent; Brinell hardness, 65-68.

In comparison with sheet produced by rolling a magnesium alloy of the same composition by conventional practice, the sheet produced by our new method not only is flat, but also shows on an average as much as 10 to 12 per cent greater yield strength and about 8 per cent greater hardness.

Other modes of applying the principle of our by a high yield strength and freedom from waviness which consists in rolling the metal to nearly the final thickness at a temperature between 850 and 400 F., subjecting the rolled metal to a heat treatment at a temperature between about 500 and 850 F., and finally further rolling the heat treated metal to the final thickness at a temperature between about 40 and 200 F.

2. The method of producing rolled products of magnesium and magnesium alloys characterized by a high yield strength and freedom from waviness which consists in rolling the metal to nearinvention may be employed instead of the 0118;. tly the final thickness at a temperature between explained, change being made as regards the method and the steps herein disclosed, provided those stated by any of the following claims or their equivalent be employed.

We therefore particularly point out and distinctlyclaim as our invention:-

1. The method of producing rolled products of magnesium and magnesium alloys characterized 850 and 400 F., subjecting the rolled metal to a heat treatment at a temperature between about 600 and 800 F., and finally further rolling the heat treated metal to the final thickness at a temperature between about 40 and 200 F.

ROBERT D. LOWRY. FRED L. RE'YNOLDS. 

